A seal of this kind can be used for e.g. sealing of a gap between a shaft and a housing for a pump, compressor or centrifuge, the pressure of the fluid situated inside the housing being frequently higher than the pressure of the atmosphere surrounding the housing. In such circumstances it is difficult to achieve a seal which has effective sealing properties and also gives small friction power losses. It is known to use sealing devices comprising sealing bodies made of resilient material which are pressed against a sliding surface by the pressure of the fluid to be sealed. Such a design very effectively prevents escape of the pressurized fluid, but friction losses and wear are considerable if the overpressure is great, and if the pressure ceases the sealing ability decreases or disappears. Lubrication may cause less power loss, but the environment is not always such that an efficient lubricant can be used. For example, lubricating oil must not be used in food processing machines where the oil might come into contact with the food. It is also known, e.g. from U.S. Pat. No. 3,880,434, to separate two spaces by introducing a pressure medium in a gap between two relatively rotatable surfaces, which gap communicates with said spaces. The pressure in the device is low and about equal in both spaces, and a pressure medium is supplied in great quantities to the gap, whereby a good separation of the spaces is assured as long as the pressure medium flows through the gap and out into both spaces. The consumption of pressure medium is, however, great if there is a great difference in pressure between the supplied pressure medium and the fluids situated outside the gap. If the supply of pressure medium is interrupted, the sealing function becomes uncertain. Only the resilient pre-stress of a bellows seal carrying one of the surfaces defining the gap can possibly contribute to closing the gap, thereby providing a sealing effect.
Whether this actually occurs is determined by the tension force in the bellows, the pressure difference, the design of the elements being provided with the surfaces defining the gap, etc.
The DE patent application No. 2153403 shows a pressure fluid operated seal comprising two parallel elastic annular membranes between which a pressure medium is supplied to a gap between two relatively rotatable surfaces. The seal comprises a device which prevents too fast an escape of pressure medium between the membranes and out of the gap. This arrangement is complicated and expensive, since it requires a special design of both the shaft and the surrounding housing.
The purpose of the present invention is to provide a seal of the kind stated above, which seal can be made of simple and inexpensive parts and be designed always to give a low consumption of the pressure medium supplied to the device. To this end, the seal comprises a pair of spaced flexible membranes connected to one of the members and an element disposed between the members, the element and other member having confronting faces defining a gap therebetween, means for supplying a pressurized fluid to the space between the membranes and to a pocket formed in one of said faces to provide a fluid seal between said confronting faces, said space defined by rigid walls of said one member and element and flexible membranes in a predetermined configuration so that the fluid pressure in said space deflects said membranes and said element in a direction tending to close the gap.
A device according to the invention requires a conventional shaft having one simple flange, and the interior of the surrounding house can be easily manufactured to the proper shape.